1. Field of the Invention
The present invention relates to a video tape control time code reading method and apparatus. Embodiments of the invention may be incorporated into video tape recorders (VTRs) to enable dual-standard VTRs to be made. Such a dual-standard VTR is capable of distinguishing recorded video signals having a frame frequency of 30 frames per second, such as in the National Television System Committee (NTSC) system, and recorded video signals having a frame frequency of 25 frames per second, such as in the International Radio Consultative Committee (CCIR) system.
2. Description of the Prior Art
The editing of video tape is of substantial importance, particularly for persons concerned with the production and broadcasting of television programs. While such editing can be done by cutting and splicing the video tape, this method is extremely tedious and time-consuming, involves considerable risk of irreparably damaging the video tape, and substantially degrades the quality of signal reproduction which can be obtained from the edited video tape. So-called electronic editing is therefore now preferred. In this method signals are selectively recorded on a single video tape from more than one source, or an existing signal on a video tape is edited by erasing parts of the recorded signal and recording substitute parts derived from another source. This can be done without introducing any discontinuity into the edited recorded signal which would be noticeable upon subsequent reproduction. A critical factor in such editing is the ability to rapidly locate specific parts of a signal, most particularly individual frames. For this purpose the Society of Motion Picture and Television Engineers (SMPTE) in the United States of America and the European Broadcasting Union (EBU) in Europe have proposed respective so-called control time codes for recording on a video tape, for example, on the number 2 audio track or cue track. The SMPTE control time code will be described in detail below. The EBU control time code is very similar to the SMPTE control time code, most of the differences not being material to the present invention. The essential feature of each of the control time codes is that it comprises in binary coded form a time, which may be the time of day, expressed in hours, minutes and seconds, and within each second, individual frame numbers of the recorded video signal. In other words, each frame of the recorded video signal is uniquely identified.
These control time codes, and VTRs able to read and be controlled making use of these control time codes, enormously facilitate editing, but there is a problem, at least for professional users, that different video tapes bearing recorded signals corresponding to different systems, and in particular systems operating at different frame frequencies, need to be distinguishable to ensure appropriate decoding and correct reproduction.